Inertial stabilization in gradiometer systems is typically handled either passively, such as in fixed-base systems with mechanical isolators, or by using mechanical gyroscopes as rotational sensors on a three gimbal stabilized platform separated from the gradient sensors by passive mechanical isolators. Systems designed to be mounted on moving bases have been stabilized using gimbal structures often referred to as “outside-in” structures. FIG. 1 illustrates such a gravity gradiometer instrument (GGI). An outside-in structure is characterized by a system of gimbals which surround the instrument package 120 and define axes of rotation (X,Y,Z) which enable the instrument package 120 to be maintained in a stable position. Each level of gimbal envelops the previous gimbal with the instrument package 120 positioned in an innermost area of the system. The instrument package 120 is housed within an innermost housing 130. The innermost housing 130 is rotatable about a spin axis Z. Innermost housing 130 may be connected to a shaft (not shown) coupled to a servo which provides rotation of the innermost housing 130 along the spin axis Z based on a rotational sensor which provides a signal indicative of a position of the instrument package 120 relative to the spin axis Z. The shaft provides a first degree of rotational freedom and serves as the first level of gimbal. The instrument package 120, includes accelerometers A1, A2, A3, attached thereto, and is located within the inner most housing 130 Innermost housing 130 is located within a first rotational housing 150. The first rotational housing 150 has a bearing 140 mounted on each side of innermost housing 130 providing a second level gimbal which provides rotational motion along input axis Y. The input axis Y is orthogonal to the spin axis Z and provides a second degree of rotation to the instrument package 120. First rotational housing 150 is operatively coupled to a servo which provides rotational movement of the first rotational housing 150 along input axis Y. A second rotational housing 160 surrounds the first rotational housing 150. First 150 and second 160 rotational housings are connected through bearings which are concentric along input axis X and provide rotational movement about input axis X. The second rotational housing 160 is operatively coupled to a servo which provides rotational motion of the second rotational housing about input axis X, which is orthogonal to input axis Y as well as spin axis Z. Bearings (not shown) disposed within the wall of second rotational housing 160 and first rotational housing 150 serve as a third gimbal providing another degree of rotation for instrument package 120. With rotational axes X, Y and Z mutually orthogonal to one another, instrument package 120 is provided with three degrees of rotational freedom, thereby allowing instrument package 120 to remain in a fixed position with respect to that which is outside of the vehicle carrying the GGI.
Such a mobile GGI typically has three degrees of rotational isolation, with the instrument package 120 comprising a plurality of accelerometers A1-A4, being supported by and located within, one or more housings. In such a configuration, the accelerometers A1-A4 are affixed to a structure 120 within the GGI having an outside dimension smaller than that of the surrounding structures providing the further levels of gimbals. Because the accelerometers are housed within the housing having the smallest outside dimensions, the distance between the accelerometers is limited by these dimensions. Smaller distances between accelerometers in a GGI reduces the accuracy of the gravity gradient readings obtained from the GGI.
Non-stationary GGIs are inertially stabilized at least in part due to the inherent problems that arise when attempting to measure gravity gradients in a rotating measurement platform. As gradiometer systems increase in size (e.g. to achieve greater accuracy) larger stabilization systems are required. Practical limitations on the size of systems capable of achieving stabilization levels required for gradiometry and space limitations placed on many applications create an upper bound of available configurations providing the required performance.
The accuracy of a GGI is dependent on the distance separating the various acceleration sensors used to determine the gravity gradients. This accuracy may be compromised by unknown rotations of the gradiometer instrument. Therefore, moving base gradiometer measurements are performed from an inertially stabilized instrument platform. When mechanical gyroscopes (gyros) are used as rotational sensors to stabilize the platform, they need to be vibrationally isolated from the gradiometer instrument. Balanced against this need is the requirement that the rotational rates of the instrument itself need to be controlled and/or determined, rather than that of the platform. If optical gyroscopes are used, quantization effects may lead to performance problems for jitter control that render such devices inadequate for gradiometer applications. Optical gyros with reduced quantization (longer optical paths) may be used, however, the cost of such devices is often too great to implement in gradiometer systems.
Alternative systems and methods are desired.